Scanning interferometry is used to gain information about a test object. Information about, for example, the surface structure can be relevant to flat-panel display (FPD) metrology, e.g., the characterization of FPD components, semiconductor wafer metrology, and in-situ analysis of thin films and dissimilar materials. Examples of relevant information include besides the surface topography itself, features of a complex surface structure, such as thin film parameters (thickness or index of refraction), discrete structures of dissimilar materials, and discrete structures that are under-resolved by the optical resolution of an interference microscope.
Interferometric techniques are commonly used to measure the profile of a surface of an object. To do so, an interferometer combines measurement light reflected from the surface of interest with reference light reflected from a reference surface to produce an interferogram. Fringes in the interferogram are indicative of spatial and structural variations between the surface of interest and the reference surface.
A scanning interferometer scans the optical path length difference (OPD) between the reference and measurement light of the interferometer over a range comparable to or larger than the coherence length of the interfering light. For multiple scan-positions, a detector measures the intensity of the interfering light, which is the basis for a scanning interferometry signal (hereafter also interferometry signal). For surface interferometry, for example, multiple camera pixels can be used to measure a spatial interferogram at each scan position, with each camera pixel measuring an interferometry signal for a corresponding location of the test surface over the range of scan positions. An interferometry signal is typically characterized by a sinusoidal carrier modulation (the “fringes”) with bell-shaped fringe-contrast envelope.
A limited coherence length of the interfering light can be produced, for example, by using a white-light source, which is referred to as scanning white light interferometry (SWLI). A typical SWLI signal features a few fringes localized near the zero OPD position which is defined as an equal optical path length for the reference and measurement light.
The conventional idea underlying interferometric metrology is to derive features of an object from the interferometry signal. The analysis can be performed in a scan domain, i.e., using the interferometry signal depending on the scan-coordinate, or in a frequency domain, i.e., using an interferometry spectrum derived from the interferometry signal.
For surface profiling, the first approach includes, for example, to locate the peak or center of the envelope, assuming that this position corresponds to the zero OPD of a two-beam interferometer for which one beam reflects from the object surface. The second approach includes, for example, calculating the rate of change of the phase of the transformed interferometry signal with the wavelength, assuming that an essentially linear slope is directly proportional to a surface height of the test object. This latter approach is referred to as Frequency Domain Analysis (FDA). See, for example, U.S. Pat. No. 5,398,113, U.S. Pat. No. 7,106,454, U.S. Pat. No. 7,271,918, the contents of which are herein incorporated by reference.
Conventional techniques used for surface characterization (e.g., ellipsometry and reflectometry) rely on the fact that the complex reflectivity of an unknown optical interface depends both on its intrinsic characteristics (material properties and thickness of individual layers) and on three properties of the light that is used for measuring the reflectivity: wavelength, angle of incidence, and polarization state. In practice, characterization instruments record reflectivity fluctuations resulting from varying these parameters over known ranges.